Original ArticlesEffects of addition of rare earth element Gd on the lamellar grain sizes of a binary Ti-44Al alloy
Introduction
Two-phase near γ TiAl alloys have been developed in the last two decades for high temperature structural applications [1]. The alloys with the fully lamellar structure have been shown to possess high fracture toughness and creep resistance although their room temperature ductility and strength are generally not as good as those shown by the materials with the duplex structure 2, 3. Recently, it has been shown that the ductility of the alloys can be improved considerably by refining the lamellar grain sizes and their strength can be increased dramatically by reducing lamellar spacings [4]. The strength may remain high at elevated temperatures if the fine lamellar structure can be retained [5]. The fracture toughness and creep resistance of such materials are also enhanced 4, 6.
The refined grain sizes and/or lamellar structures have been achieved by various methods [7] including thermomechanical treatment, novel heat treatments, addition of boron, and extrusion at temperatures above the α transus temperature. In casting TiAl alloys that cannot be deformed, additions of boron in excess of 0.5–1 at% have been shown to be effective in refining the lamellar grain sizes 8, 9, 10.
Rare earth (RE) elements have been known to refine the grains in materials such as steels, cast irons, aluminium alloys, titanium alloys and magnesium alloys [11]. Additions of RE elements to FeAl [12], Fe3Al [13], NiAl [14] and TiAl [15] were shown to increase the ductility and strength of the alloys at room temperature. However, the microstructures of the fully lamellar TiAl alloys with additions of RE elements have not been investigated in detail. In the present work the effects of adding the RE element Gd on the refinement of lamellar grains in a binary Ti-44Al alloy were studied. The microstructures after casting and/or hot pressing followed by various heat treatments are presented.
Section snippets
Experimental materials and procedures
A binary alloy Ti-44 at% Al and several alloys based on it but containing the rare earth element gadolinium (Gd) were made by arc melting in a vacuum chamber. The titanium and aluminium metals used were in the forms of cut bars and ingot and of purities of 99.9 wt% and 99.99 wt%, respectively. The gadolinium used was in the form of chips and of purity of 99.5 wt%. The alloys were remelted five times to ensure the uniformity of the compositions. The ingots were eventually drop cast into a square
As-cast structures
The as-cast macrostructures of the Ti-44 at% Al, the Ti-44 at% Al-0.15 at% Gd and the Ti-44 at% Al-0.5 at% Gd alloys are shown in Fig. 1. In all the three materials there were the normal three zones of solidification, i. e. the chill zone of very fine equiaxed grains at the surface, the columnar zone with elongated grains growing from the surface, and the central zone of coarse equiaxed grains. The columnar zone was most significant in Ti-44 at% Al-0.5 at% Gd and was the shortest in the binary
Discussion
For the binary alloy the columnar zone was not well defined and the primary solidification was reported to involve the formation of the β phase [16]. The columnar zones, however, were clearly seen in the Gd containing materials, especially in the alloy with 0.5 at% Gd. It has been shown in some studies (e. g. [8]) and in our own investigation that well-defined columnar zones formed in Ti-48 at% Al alloys. Also, the individual columnar grains consisted of laths with the same orientations
Summary
Rare earth element Gd can significantly reduce the lamellar grain sizes in as-cast Ti-44 at% Al alloys. The grains were refined to ∼400 μm with an addition of 0.15 at% Gd. Hot-pressing by ∼15% in height reduction only led to marginal grain refining in the binary alloy whereas the deformation resulted in dramatic reduction in grain sizes to ∼140 μm in the Gd containing alloy. The grain sizes are considerably more stable in the alloys containing Gd.
Acknowledgements
We are grateful to Dr. A. Morton, Mr. D. Jones and Mr. R. Allen of the Division of Manufacturing Science and Technology of CSIRO and Mr. X. Wu of the University of Melbourne for their support and help in melting and hot pressing the materials. This project was partly funded by the Australian Research Council and a University of Melbourne special grant.
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